Deformation process for producing stress relieved metal/ceramic abradable seals

ABSTRACT

Metal/ceramic abradable seals of the type used in jet aircraft are improved by using a substrate which is able to deform during cooling from the brazing temperature and then rendering the substrate rigid after the cool down.

This invention relates to metal/ceramic abradable seals of the type usedin jet aircraft. More particularly it relates to a method of making suchseals in which stresses are minimized during cooling of the sealstructure after fabrication.

A graded metal-to-ceramic structure for high temperature abradable sealapplications is described and claimed in U.S. Pat. No. 3,975,165,together with a method of making it. Seals made according to theteachings of this patent contain appropriately formulated mixtures ofmetal and ceramic between the ceramic surface of the composite sealmember and the substrate to which it is fused, whereby thermal expansiondifferences are reduced. As a result of the graded construction shearstresses, which would likely cause failure of the seal, aresignificantly reduced.

Despite this reduction in shear stresses, however, residual stresses mayremain large. For example, a nichrome-ZrO₂ structure isothermally cooledfrom bonding temperatures may develop residual compressive and tensilestresses in the ceramic and metal-rich layers, respectively. When thegraded composite structure is attached to a stiff or rigid substratesuch stresses occur and can build up rapidly. The dimensionally stablestructure required for most applications therefore, is certain todevelop large residual stresses after processing.

An object of the present invention therefore, is to prepare soundceramic/metal composite structures which are abradable and resistant tothermal shock. A further object is to prepare such structures in asimple economical manner which reduces stress in the final products. Yetanother object is to provide in such manner a graded metal/ceramicstructure capable of sustaining cyclic heating and cooling rates of atleast 50° C. per second.

According to our invention, metal/ceramic abradable seals of the typeused in jet aircraft are improved by sintering the seals while at thesame time brazing or bonding them to a deformable substrate and thenrendering the substrate rigid after cooling from the brazingtemperature.

In the operation of our invention the unsintered graded metal/ceramiccomposite formed, compressed and then dried slowly. The thus formedcomposite is placed in the unsintered state on a deformable substrate,which preferably has a braze tape or fused coat of braze powder, andheated to a temperature sufficient to sinter the composite and braze orfuse the metal-rich surface of the composite to the substrate. Atemperature of about 1200° C. is preferred for this brazing operationand is maintained for a sufficient time, typically about one hour. Thisheating step can be isothermal or a slight temperature gradient can bemaintained, with the substrate up to 450° C. cooler than the ceramicsurface of the composite. Preferably this heating is done in a vacuum ornonoxidizing atmosphere such as hydrogen. It is also preferred thatmoderate pressure be applied to force the composite and substratetogether with about 5 psi having been found satisfactory.

When the temperature of these structures is near the braze temperatureof about 1200° C. the pressure is preferably raised to from 5 to 225 psiin order to improve contact at the substrate/composite interface and toheal minor structural flaws. After fusion of the graded composite to thesubstrate is complete, usually about one hour, cooling to ambienttemperature is begun. The substrate used according to the invention isdeformable and all pressure and mechanical constraints, which mightinterfere with the deformation process, are removed for the coolingcycle. After cooling to ambient temperature stiffeners and rail hooksare welded to the substrate back side to rigidify the piece andfacilitate attachment to the turbine housing.

Deformation occurs in graded metal/ceramic seal, such as a nichrome-ZrO₂layer structure because the ceramic and metal parts contract differentlywhen cooled. The deformation or displacement will decrease as thesubstrate rigidity increases. The alloys and thicknesses of thesubstrate affect rigidity and thus offset deformation as well.

IN THE DRAWING

FIG. one is a bottom or backside view of a ribbed substrate according tothe invention before it is bonded to the composite seal member.

FIG. two is a side elevation of the substrate in FIG. one.

FIG. three is a side elevation of a ribbed substrate according to theinvention after a metal/ceramic seal has been braze bonded to it.

FIG. four is a side elevation of a ribbed substrate as in FIG. three butwith rigidifying rods welded in place.

FIG. five is a bottom view of the ribbed substrate of FIG. four.

FIG. six is a graph showing the effect of substrate alloy and platformthickness on deformation of structures according to the invention.

FIG. seven is a side elevation of a fixture for holding a seal andsubstrate during furnacing.

FIG. eight is a top view of the fixture of FIG. seven.

FIG. nine is a side elevation of a furnacing fixture incorporating asegmented pressure pad and a pressure bladder.

FIG. ten is a top view of the fixture of FIG. nine.

FIG. eleven is a section through FIG. nine.

In FIG. one is seen a bottom or backside of a ribbed substrate with theplatform 10 ribs 12 and end ribs 14. The end ribs 14 have preferredslots 16 which relieve axial stresses.

FIG. two is a side elevation of the same rib substrate of FIG. one. InFIG. three a metal/ceramic seal composite 18 has been brazed to theplatform 10.

In FIG. four the seal 18 is shown brazed to the platform 10 as in FIG. 3but now rails 20 have been welded in place as circumferentialstiffeners. In addition bars 22 have been welded to the end ribs 14 tofurther stiffen the structure. FIG. 5 is a bottom view of the stiffenedstructure of FIG. four and the same elements can be seen. The graph inFIG. six showing the effect of substrate alloy and platform thickness ondeformation of the structures of the invention is self explanatory.

FIG. seven illustrates a type of fixture according to the invention forholding a seal composite and substrate during furnacing. A graphitebottom plate 30 has four graphite bolts 32 mounted on it and passingthrough an upper graphite plate 34. Graphite nuts 36 can be tightened toapply pressure downward on the structure. In FIG. 7 the basic supportfor the substrate platform 10 through its ribs 12 is a shaped stainlesssteel plate 37. Thin sheets of Inco 760 sheet 38 are positioned oneither side of the shaped stainless steel plate 37 to prevent sticking.Above the metal/ceramic seal 18 is a shaped graphite pressure pad 40.Above the pads 40 is another layer or sheet of Inco 760 sheet and abovethat a steel plate 42. Sheets of fibrous Fiberfrax 44 are used toprevent sticking and to relieve pressure on the composite after brazingas they become plastic at braze temperature and relax. In top view ofFIG. eight can be seen the upper graphite plate 34, the ends of thegraphite bolts 32 and the graphite nuts 36.

A similar but different pressure system is shown in FIG. nine. A baseplate 50 made of graphite has graphite bolts 52 positioned above it. Ashaped steel plate 54 with a Fiberfrax sheet 44 below it supports thesubstrate platform 58 and composite seal 60. Exerting pressure on thecomposite 60 are segmented graphite pads 62 which are held in a graphiteframe 64 supported on support nuts 66. A gas bladder 68 is positionedbetween the upper graphite plate 70 and the segmented graphite pads 62with layers of zirconia cloth 69 on either side of the bladder 68. A gasfitting 72 is connected to the gas bladder 68 and extends up through theupper graphite plate 70 to access an outside source of gas not shown.Top nuts 74 maintain the upper graphite plate 70 in place as pressure isapplied through inflation of the gas bladder 68.

EXAMPLE I

A six-layer graded composite was formed from the following materials:

A. 35/60 Tyler mesh ZrO₂ hollow spheres

B. 100/250 Tyler mesh. ZrO₂ broken spheres.

C. 35/60 Tyler mesh ZrO₂ agglomerate

D. 100/250 Tyler mesh ZrO₂ agglomerate

E. -325 mesh ZrO₂ powder

F. 100/250 Tyler mesh; 80% Ni, 20% Cr powder

G. AMl--400 Braze powder (made by Alloy Metals Inc.)

H. Ludox 130M (a colloidal silica solution manufactured by E. I. du PontCo.)

The composition of each of the six layers was as follows:

    ______________________________________                                                                        Percent of Total                              Layer Thickness Percent Weight of                                                                             A,B,C,D,E,F,G                                 No.   in Inches A     B   C   D   E   F   G      H                            ______________________________________                                        1     0.150     45    25  --  --  30  --  --     6.2                          2     0.025     --    --  10  20  40  30  --     7.5                          3     0.025     --    --  10  15  25  50  --     7.5                          4     0.025     --    --  20  10  10  55  5      7.5                          5     0.025     --    --  25  --  --  65  10     7.5                                          Porous sheet                                                  6     0.025     of 80% Ni and 20% Cr Powder                                   ______________________________________                                    

Layers 1 through 5 were each mixed with sufficient water and Ludox 130Mto form a damp mixture. Layer 1 was first spread in a mold to form alayer 2.40 inch by 4.05×0.150 inch thick. The mold had a radius ofcurvature of 10.97 inch. The remaining layers were stacked successivelyon top of the first and the total was pressed at 10,000 psi. The pressedcompact was carefully dried at 5° C. and 15 percent relative humidity.

The substrate used was a curved Mar M 509 alloy plate measuring 4.00inch by 3.10 inch by 0.125 inch thick. The substrate was axiallystiffened with seven ribs spaced 0.350 inch apart and having across-section measuring 0.097 inch high by 0.200 inch wide. Twoadditional ribs at each end of the platform were 0.425 inch high by0.200 inch wide with slots to the platform and spaced 0.500 inch apart.The Mar M 509 is a cobalt/chromium alloy made by Pratt and Whitney. Theconcave surface of the plate was carefully covered with AMl--400 brazetape.

Pressure bonding and sintering was done isothermally in the fixtureshown in FIG. seven. The braze-coated surface of the Mar M alloy platefaced the metal-rich surface of the graded composite and theceramic-rich surface was in contact with the concave graphite pressurepad. Special effort was taken to assure good alignment through theassembly. The graphite nuts on the restraining member are finallytightened uniformly to 2-21/2 in. lbs.

The total assembly was inserted into a gas tight muffle. The muffle wasflushed with Argon for 1 hour at 5.4 CFH. After purging, it was insertedinto the furnace idling at 760° C. Hydrogen gas was then admitted intothe muffle at a flow rate of 5.0 CFH and the Argon was lowered to 0.4CFH. The furnace controller was adjusted to achieve a 1240° C. internaltemperature. The indicated peak temperature was maintained for threehours after which the temperature was lowered to 760° C. The muffle waswithdrawn when the furnace temperature reached 1000° C.

A sound structure with the graded composite well bonded to the curvedsubstrate was obtained. Visual examination revealed no defects andmeasurements indicated deformation had occurred. The change in maximumdistance between arc and three-inch chord in the radial direction forthe substrate alone (y in FIG. two) and the seal after welding to thesubstrate (y in FIG. three), correspond to 0.025 inch. In the axialdirection minor arching occurred which was not sufficient to affect theoperability of the seal.

Two Hastelloy C-276 (a cobalt/chromium/nickel/molybdenum alloy made byStellite Division of Cabot Corporation) rails measuring 0.500 inch wideand 0.125 inch thick were employed as circumferential stiffeners. Therails were formed to match the substrate curvature. They weresymmetrically positioned on the substrate, spaced one inch apart, andlaser welded to the ribs. The slotted end ribs were also stiffened bywelding 0.375 inch wide by 0.125 inch thick flat bars. The substrate wascut to a 2.375 inch width to correspond with the composite dimension.

The specimen prepared in this example was thermally cycled 50 timesunder simulated turbine engine conditions. The severe cycle consisted ofheating the ceramic surface from ambient to 1000° C. in 15 seconds,maintaining maximum temperature of 1250° C. for 60 seconds, cooling thesurface to 600° C. in approximately 15 seconds, and repeating. Specimenintegrity was considered excellent after this test.

EXAMPLE II

A six-layer graded composite was formed using materials and proceduresdescribed for Example I, except that the substrate had no axial ribs butwas a simple curved plate or platform, a curved Hastelloy C-276 alloyplate measuring 4.06 inch×2.50 inch×0.125 inch thick.

Pressure bonding and sintering was done isothermally in the fixture ofthe type shown in FIG. nine. Furnacing procedures followed thosedescribed in Example I except the metal bladder was pressurized with 10psi argon during the three-hour hold at peak temperature. The bladderwas depressurized prior to cool-down.

A sound structure with the graded composite well bonded to the curvedsubstrate was obtained. Visual examination revealed no defects andmeasurements indicated deformation had occurred. The largest change inmaximum distance (y) in the radial direction between arc and three-inchchord corresponded to 0.025 inch. In the axial direction minor archingoccurred which was not sufficient to affect the operability of the seal.

The specimen prepared in this example was thermally cycled 50 timesunder simulated engine conditions described in Example I. Specimenintegrity was considered excellent after this test. Since the substratewas not stiffened prior to test as in Example 1, the specimen underwent"ratcheting". That is, deformation occurred after each thermal cycle. Ina specimen intended for use in an engine stiffening ribs in an axialdirection as well as circumferential rails would be welded or otherwiseattached to the substrate in order to avoid the ratcheting or twisting.

What is claimed is:
 1. Process for bonding a multi-layer metal/ceramicabradable composite to a deformable metallic substrate which comprisesplacing said abradable composite, in the unsintered state, with itsmetal-rich surface in contact with said substrate, heating saidcomposite and substrate under pressure to a sintering temperaturesufficient to sinter said composite and bond to said substrate,maintaining such a temperature until said sintering and bonding arecomplete, substantially removing said pressure, then cooling the thusformed structure to ambient temperature and then fastening reinforcingmembers to the backside of said substrate to rigidify it.
 2. Processaccording to claim 1 wherein the surface of said substrate is coatedwith a brazing powder prior to heating.
 3. Process according to claim 1wherein said heating to said sintering temperature and cooling areconducted in an inert atmosphere.
 4. Process according to claim 3wherein said inert atmosphere is a vacuum of at least 10⁻⁴ torr. 5.Process according to claim 3 wherein said inert atmosphere is selectedfrom the group consisting of argon, hydrogen and nitrogen.
 6. Processaccording to claim 1 wherein said pressure is at least 5 pounds persquare inch during heating.
 7. Process according to claim 1 wherein saidreinforcing members are metal bars.
 8. Process according to claim 7wherein said metal bars are welded to said substrate backside. 9.Process for producing a porous abradable seal on a turbine enginesurface which comprises:(a) forming a composite comprising a top layerof substantially all ceramic material at least one intermediate layer ofa mixture of ceramic material and a metallic bottom layer, (b) pressingthe composite in a suitable fixture under pressure and then slowlydrying the composite to thereby form a multi-layer composite, (c)placing the dried composite on a deformable metallic substrate with saidmetallic bottom layer of said composite in contact with said substrate,(d) heating said composite in contact with said substrate under pressureto a sintering temperature sufficient to sinter said composite and bondsaid composite to said substrate and maintaining such a temperatureuntil said sintering and bonding are complete, (e) substantiallyremoving said pressure on the structure formed in step d, and thencooling said structure to ambient temperature whereby said metallicsubstrate assumes the desired final configuration, and (f) fasteningreinforcing members to the backside of said substrate to rigidify it.10. Process according to claim 9 wherein step c the metallic substrateis first coated with a brazing powder prior to the placing of thecomposite on it.
 11. Process according to claim 9 wherein steps d and eare conducted in an inert atmosphere.
 12. Process according to claim 11wherein said inert atmosphere is a vacuum of at least 10⁻⁴ torr. 13.Process according to claim 11 wherein said inert atmosphere is selectedfrom the group consisting of argon, hydrogen and nitrogen.
 14. Processaccording to claim 9 wherein said pressure is at least 5 pounds persquare inch during step d.
 15. Process according to claim 9 wherein saidreinforcing members are metal bars.
 16. Process according to claim 15wherein said metal bars are welded to said substrate backside.